Understanding the Solution-Phase Catalysis Process inside the Li-O2 Battery Using Redox Mediator─Butylated Hydroxytoluene.

Abstract

The redox mediators help prevent cathode passivation and promote the formation and decomposition of Li₂O₂ within the electrolyte of the battery. Understanding the mechanistic properties of the soluble catalyst from an atomic level is crucial for developing an all-in-one multifunctional soluble catalyst for Li-O₂ batteries. With the help of density functional theory and atom-centered density matrix propagation molecular dynamics simulations, we report how butylated hydroxytoluene (BHT), an experimentally reported soluble catalyst, mediates the stabilization of reactive intermediates and the mechanism behind the formation and decomposition of Li₂O₂. The hydroxy group in BHT facilitates the stabilization of O₂^•- via hydrogen bonding and the solvation of Li⁺, LiO₂^•, and Li₂O₂. This characteristic of BHT helps to promote the solution-phase mechanism and suppress parasitic reactions induced by O₂^•-. During the charging process, the reversibility of BHT and BHT^•+ happens and the disappearance of the hydrogen bonding interaction facilitates the delithiation process. The Mulliken charge distribution analysis shows that the reversibility of BHT and BHT^•+ is due to the electron delocalization between the oxygen atom and benzene ring of BHT. We observe the two benefits of the hydrogen bond: the presence and absence of hydrogen bonding enhance the formation and decomposition of Li₂O_2, respectively. We find that tetraethylene glycol dimethyl ether solvent plays a significant role in stabilizing lithium-oxygen-containing species such as LiO₂^• and Li₂O₂. However, the presence of BHT further improves the results. This finding highlights the cooperative activity of BHT in conjugation with the tetraethylene glycol dimethyl ether solvent. The atom-centered density matrix propagation method reveals that BHT facilitates Li₂O₂ decomposition through protonation, whereas BHT^•+ induces Li₂O₂ decomposition by promoting the formation of LiO₂^• and the BHT:Li⁺ complex without transferring the proton.